The movement of solutes from the maternal plasma to the fetus has been studied with 13 pregnant guinea pigs near term. The transplacental rates of transfer of labeled water, mannitol and polyethylene glycol (molecular weight 1000) have been determined. Hyperosmotic solutes mannitol and PEG 1000 have been used as driver solutes in solute pairs in which radiolabeled mannitol and PEG 1000 have been used as the tracer or driver solute. Only preliminary results are available because of the necessary evolution of a final perfusion and analysis system. It does appear that the pregnant guinea pig preparation can be used to determine if solute drag can influence transplacental exchanges. In the most recent experiments PEG 1000 appeared to be a better driver of the tracer solutes than was mannitol. This is in keeping with principles previously established by us on in vitro, non-biological membrane systems.